The absorption dryer is one of several types of dryer. Two other important types are the adsorption dryer and the condenser dryer. The adsorption dryer operates with a solid drying agent, which is usually regenerated with the aid of electrically heated hot air. The adsorption dryer frequently consists of a fabric structure similar to corrugated fibreboard and impregnated with a drying agent. This structure usually is in the form of a rotor, and the humid air is blown through the flutes and dehumidified. The drying agent can be regenerated by a sector of the rotor being reserved for the regenerating function. The sector is sealed in some suitable manner on both sides of the rotor, and hot air is blown through the rotor material which at that very moment is located in the sector. The advantage of such an adsorption dryer is that it can dehumidify down to low relative moisture contents, since solid drying agents, as a rule, have very low vapour pressures of equilibrium with water vapour in, for instance, air. 10-30% RH thus is quite possible to achieve in exit air, but if supply air is not too humid, suitably not exceeding 50-60% RH.
Although the adsorption dryer must be considered relatively simple in respect of design, it must, however, comprise a driving motor and a comparatively complex rotor. Its great disadvantage is, however, its lack of operation economy. By operation economy is meant the power requirements in kWh per kg of removed water. 1.2-1.5 kWh per kg of water is reasonable for an adsorption dryer. The theoretical power requirements are about 0.75 kWh per kg of water. The reason for the losses in the adsorption dryer is that the rotor, during its continuous rotation between the cold and the warm sector, consumes a considerable amount of power during the exchange of heat. Moreover, the adsorption dryer is not very convenient at high moisture contents and temperatures, or when large amounts of water are to be removed, such as in the case of damage by water or in operating situations in moist and warm climates, including laundry rooms. A further disadvantage of the adsorption dryer is that it is difficult to exchange the heat of warm and humid regeneration air in an inexpensive fashion. In other words, a gas/gas heat exchanger is necessary, and therefore usually just the warm and humid regeneration air is released outdoors. Otherwise, by heat exchange/condensation of moisture it is possible to recover the supplied power as heat, thereby avoiding wall ducts.
The condenser dryer, is built more or less like an ordinary refrigerating or freezing machine. Humid air is dehumidified by contacting the cold surface of the evaporator. The condenser dryer has good operation economy and can process air having a high moisture content and a high temperature. The supplied power remains in the room, and humid hot air need not be wasted outdoors. However, the condenser dryer suffers from the drawback that a compressor having a driving motor is necessary. In continuous operation, the life of the most expensive components of the dryer will be just about a year. A further drawback of the condenser dryer is that it contains FREON fluorocarbon, which, as is well known, is a much discussed substance these days from the environmental point of view. Besides, the condenser dryer is relatively expensive, possibly excluding the smallest orders of size, which can use components manufactured in very large series for the refrigerator industry.
The absorption dryer requires good phase contact between a liquid drying agent and the humid gas. This can be accomplished in a number of ways. A common technique is to establish such phase contact in a column containing fillers. As a rule, the drying agent is sprayed over the column packing at the head of the column, while the humid air is introduced countercurrently at the bottom of the column. A problem in columns is that it is difficult for this construction, owing to its geometry, to dispose of the heat of reaction by thermal radiation. In fact, considerable amounts of heat are generated when water vapour from the humid gas is absorbed in the liquid drying agent, which besides also applies to the adsorption dryer. A higher operating temperature and lower efficiency must thus be accepted, since efficiency decreases as the operating temperature rises. Alternatively, refrigerating coils can be arranged in the column packing, or the liquid drying agent can be removed at several levels, then be cooled and be fed back again. Another technique of avoiding too great a rise in temperature is to use highly intensive pumping-around of the drying agent through the column and through an external cooler. Part of this flow is then concentrated continuously in a boiler.
A further common construction is based on the spray-drying technique. A fine mist of the liquid drying agent encounters the humid air countercurrently in a tubular reactor. In this construction, it will be very difficult to have the dry exit air completely free from droplets of the drying agent, and moreover the thermal problems are the same as in the preceding case.
A third variant of the absorption dryer is a contact means having the structure of corrugated fibreboard, flutes extending in two directions perpendicular to each other, the material being permeable to liquid but impermeable to gas. The liquid drying agent can be sprinkled on a body of the material so as to flow down through the vertical flutes, and air can be injected through the remaining horizontally oriented flutes. Also in this case the thermal problems or restrictions as mentioned above will appear. Otherwise, the principle uses the condition of concentrated liquid drying agent permeating the walls of the air flutes, encountering humid air and absorbing water, which diffuses towards a more concentrated drying agent solution in the adjoining drying agent flutes. This technique, however, has its limitations, viz. a limitation of the diffusion of the wall material in the corrugated structure. The geometric interface per unit of volume will also be relatively small, which results in a low capacity per unit of volume of corrugated contact material.